A filter is often constituted by a MOSFET-C filter or a Gm-C filter. The reason is that the MOSFET-C filter and the Gm-C filter are filters using no operational amplifiers and therefore speeding up can be achieved.
In the above-mentioned analog filter, however, there exists a problem that the filter characteristic might be degraded due to variations in element values constituting the filter, or variations in temperature. For example, in a case of a low-pass filter, a cutoff frequency as a characteristic frequency of this filter has an error between its design value and its actual value. In order to resolve this error, conventionally, a PLL circuit including a voltage-controlled oscillator (hereinafter referred to as a VCO) is provided in a device having a filter, in addition to the filter, and the filter characteristic is adjusted by a so-called master/slave system using the PLL circuit (for example, refer to Non-Patent Document 1: “K. Tan and P. R. Gray, Fully Integrated Analog Filters Using Bipolar-JFET Technology, IEEE JSSC, December 1978”), or the characteristic of the filter is adjusted using the filter itself that is used for actual operation (for example, refer to Patent Document 1: Japanese Published Patent Application No. 2000-59162 (Pages 4-7, FIG. 1)).
Hereinafter, the conventional filter characteristic adjusting method will be described.
In a conventional master/slave system filter characteristic adjusting apparatus 300, a filter 10 is placed on the slave side, and a VCO 61 included in a PLL circuit 60 is placed on the master side, as shown in FIG. 15. The PLL circuit 60 on the master side comprises a VCO 61 which oscillates based on a reference clock s62, an oscillation frequency measurement unit 62 which measures a frequency of an oscillation signal s64 that is outputted from the VCO 61, and a control unit 63 which generates a control signal S61 for controlling the filter 10 on the basis of an output signal from the oscillation frequency measurement unit 62. It is assumed that the oscillation frequency of the VCO 61 and the characteristic frequency of the filter 10 are proportional to each other. In the filter characteristic adjusting apparatus 300 having such construction, in the PLL circuit 60 on the master side, the oscillation waveform s64 of the VCO 61 is input to the oscillation frequency measurement unit 62, and the control unit 63 generates a control signal s61 for controlling the characteristic frequency of the filter 10 on the basis of the information of the frequency that is measured in the measurement unit 62, and the GM values of the VCO 61 and the filter 10 are controlled with the control signal s61, thereby realizing automatic adjustment for the frequency characteristic of the filter 10 (for example, refer to the Non-Patent Document 1).
Further, a conventional filter characteristic adjusting apparatus 400 using a filter itself which is used for normal operation comprises, as shown in FIG. 16, a step signal generation unit 77 for generating a step signal s74 to be used for a tuning operation of the filter 10, a signal selection unit 71 for selecting a signal to be inputted to the filter 10 on the basis of a selection signal s72, a response waveform period measurement unit 74 for counting a filter output signal s76 outputted from the filter 10 to measure a frequency of the filter 10, and a control unit 78 for generating a characteristic frequency control signal s71 that controls a measurement signal s78 outputted from the response waveform period measurement unit 74 so as to be within the range of the reference frequency. Since the filter 10 is used for both the normal operation and the tuning operation, in the filter characteristic adjusting apparatus 400, the signal selection unit 71 selects a normal signal s73 as an input signal during the normal operation of the filter 10, and inputs this signal to the filter 10, whereby the filter 10 outputs a filter output signal s76 that is a response waveform to the filter input signal s75. On the other hand, during the tuning operation of the filter 10, the step signal s74 is directly input to the filter 10, and the period of the response waveform of the filter 10 to the step signal s74 is measured by the response waveform period measurement unit 74, and the characteristic frequency of the filter 10 is automatically adjusted to a desired frequency by the control unit 78 (for example, refer to the Patent Document 1).